Posttraumatic Stress Disorder is currently understood to arise from a past event. Indeed, people’s PTSD symptoms relate to their memory of a negative event. For instance, the more emotional, frequently rehearsed, and “central” their memory of the event is, the worse their symptoms. Yet people can also develop PTSD-like symptoms before an anticipated negative event. Where do these “PreTSD” symptoms come from? Across a series of studies, we found that when people imagine a negative event happening to them in the future, the more emotional, frequently rehearsed, and “central” that event is, the worse their “PreTSD” symptoms. In other words, the mental characteristics of events that lie in the past and of events that lie in the future are similarly associated with how troubling those events are. These findings suggest PTSD-like symptoms stem from the mental representation of a negative event—regardless of whether it has happened.